In a semiconductor process, in order to introduce material gas or the like into a vacuum chamber at a constant flow rate, a flow rate controller such as a mass flow controller, and a pressure controller for introducing the material gas or the like into the vacuum chamber at a constant pressure are used.
For example, the flow rate controller is one that performs flow rate control so as to enable the material gas or the like to be introduced into the vacuum chamber at a desired flow rate by controlling an opening level of a flow rate control valve such that a measured value outputted from a flow rate sensor provided in a flow path through which fluid flows becomes equal to a preset flow rate value setting.
Meanwhile, in the case of using such a flow rate controller to continuously perform the flow rate control of the material gas, a flow rate error between the measured value of the flow rate sensor and a flow rate of fluid actually flowing may gradually increase because of, e.g., an influence of a component contained in the material gas, the attachment of contamination to a flow path in the flow rate controller or a measuring part of the flow rate sensor, or other reason.
Therefore, in order to prevent a large error from occurring in the flow rate control, a flow rate controller described in Patent Literature 1 is provided with a diagnostic mechanism for verifying whether or not an error between a measured value outputted by a flow rate sensor and an actual flow rate is sufficient to have an allowable degree of accuracy.
More specifically, the diagnostic mechanism is one that, on the basis of a change in flow rate of fluid that, in a state where a valve provided in a flow path is fully closed, flows out of a reference volume, which is a volume of a flow path space from the valve to the flow rate sensor, verifies the flow rate sensor. That is, first, as illustrated in a graph of FIG. 14, the diagnostic mechanism fully closes the valve, and from a mass flow rate integrated value throughout a fluid parameter changing interval in which a flow rate changes until a pressure of the fluid changes from a preset high pressure Ph to a low pressure Pl (a measured flow rate changes from 90% to 10%), and a gas state equation, the diagnostic mechanism calculates a diagnostic volume from which the fluid is considered to have flowed out. The diagnostic mechanism is configured to subsequently determine whether or not a difference between the calculated diagnostic volume and the preset reference volume is equal to or less than an allowable value, and in the case where a difference equal to or more than the allowable value appears, determine that the measured value outputted from the flow rate sensor has abnormality.
That is, as illustrated in FIG. 14, the conventional diagnostic mechanism collectively obtains the flow rate integrated value substantially throughout the fluid parameter changing interval to calculate the diagnostic volume after having fully closed the valve, and can therefore verify whether or not the measured value has abnormality throughout a measurement range.
Meanwhile, in the past, without separately using a flow rate sensor serving as a reference, on the basis of a diagnostic volume, it has been verified whether or not a measured value of a flow rate sensor as a verification target has abnormality; however, a specific technique has not been known regarding, in the case where there is abnormality, what sort of calibration should be performed on a measured value. For this reason, in the case where it is verified to be abnormal, some operation is performed, such as stopping a semiconductor manufacturing process to do maintenance work on the flow rate controller.
In recent years, it has been required to be able to obtain a reliable measured value from a flow rate sensor even without doing such maintenance, and eliminate wasted time for the above-described maintenance or the like. Accordingly, a technique for performing self-calibration on the basis of a measured value of the flow rate sensor without separately using a sensor serving as a reference, while keeping the flow rate sensor or the like in processing equipment, has been required.
On the other hand, Patent Literature 2 discloses a mass flow controller adapted to be able to calibrate a flow rate measured value outputted from a controlling flow rate sensor by providing, separately from the controlling flow rate sensor that outputs the measured value for flow rate control, the following: a reference pressure sensor that is not used for the flow rate control and serves as a comparison target, and a tank for storing fluid for the calibration.
In this mass flow controller, a value obtained by dividing a value, which is obtained by multiplying a differential pressure between respective pressures measured by the reference pressure sensor at time Tc when a valve is fully closed and Te when the flow rate becomes substantially constant by a volume V of the tank, by an integrated value of the flow rate measured by the controlling flow rate sensor between the time Tc and the time Te is set as a calibrating parameter A. Further, the mass flow controller calibrates the flow rate measured value of the controlling flow rate sensor on the basis of a ratio between a calibrating parameter Ai at the normal time and a calibrating parameter Af calculated with use of a current measured value.
However, although a technique of Patent Literature 2 enables the controlling flow rate sensor to be calibrated, the reference pressure sensor and the tank, which are completely irrelevant to the flow rate control that is an original function of the mass flow controller, should be provided, and therefore the mass flow controller is increased in size, which makes the mass flow controller awkward to use in a semiconductor manufacturing process in which a footprint should be made as small as possible. Also, a complicated operational expression as described above is used, and also the calibration is performed on the basis of a plurality of measured values, so that an influence of an unexpected error component included in a measured value is likely to appear in the calibration, and therefore it is difficult to obtain desired calibration accuracy.
As a result of extensive examination of such technical problems by the present inventor, in the past, it has been considered that even in the case of using the diagnostic volume calculated from the integrated value of a measured value in the fluid parameter changing interval as disclosed in Patent Literature 1, although it can be verified whether or not the flow rate sensor has abnormality, it is impossible to use the diagnostic volume to perform calibration so as to output a correct measured value from the flow rate sensor; however, the present inventor has first found that the diagnostic volume calculated on the basis of the integrated value of the flow rate measured in the fluid parameter changing interval can actually be used to perform the calibration.
In other words, as a result of study, the present inventor has found that the diagnostic volume that is calculated from the flow rate integrated value in the fluid parameter changing interval and has been considered to be used only for verification has a relationship with the magnitude of an error between the measured value outputted from the flow rate sensor and a true flow rate and also has a value usable for the calibration.
Also, as illustrated in FIG. 14, after having fully closed the valve, the conventional diagnostic mechanism collectively obtains the flow rate integrated values substantially throughout the fluid parameter changing interval to calculate the diagnostic volume, and therefore although being able to make an evaluation throughout the measurement range of the flow rate sensor, cannot verify which measurement interval range causes an unallowable measurement error to occur in a measured value. In addition, which measurement interval of the flow rate sensor causes what level of flow rate error to occur cannot be quantitatively known, so that, for example, only an offset correction can be made throughout the measurement range, and therefore it is difficult to improve measurement accuracy by performing finer calibration.